The Ahr capacity of a battery
depends on the discharge rate. The most meangful number is probably the AHr capacity at a normal discharge rate. To measure his fully charge the batteries , this is important.
Disconnect the charge sources.
Then use the link to measure the AHrs out until you hit 10.5v.
If you want to match the battery
specs you do the same thing at a set discharge rate. 1/20 of the battery capacity (eg 10A for a 200Ahr battery) or whatever the battery is rated at.
Note the test will take the battery to completely flat, recharge it fully immediately afterwards and even then it will shorten the battery life. So you need a good reason to test them.